Holographic data storage can offer a high-capacity storage alternative by storing information in the volume of a medium, and not just on the surface of the medium. In some types of holographic data storage, information can be stored as an optical interference pattern within a photosensitive or other optical material.
As described in “Holographic data storage” by Ashley et al., which is incorporated by reference herein, data can be stored as an optical interference pattern in a holographic storage medium by intersecting two coherent laser beams within the storage medium. The first laser beam, called the object beam, contains the information to be stored. The second laser beam, called the reference beam, is selected to be simple to reproduce, such as a collimated beam with a planar wavefront. The resulting optical interference pattern causes chemical and/or physical changes in the holographic storage medium in a manner known to those in the art. A replica of the interference pattern can be stored as a change in the holographic storage medium (e.g., a change in an absorption property, refractive index, or thickness of the photosensitive medium). When the stored interference pattern is illuminated with one of the two waves used to store the interference pattern, at least some of the light is diffracted by the stored grating in such a way that the other wave is reconstructed. As is also well known in the art, illuminating the stored interference pattern with the reference beam reconstructs the object beam, and vice versa.
A large number of interference patterns can be superimposed in the same piece of holographic storage medium and can be accessed independently, as long as each interference pattern (also referred to herein as a “frame” of data) is distinguishable by the direction or the spacing of the interference patterns. In some cases, such separation can be accomplished by changing the angle between the object and reference beams (i.e., “angle-multiplexed”) or by changing the laser wavelength (i.e., “wavelength-multiplexed”). Any particular frame can then be accessed and read independently by illuminating the frame with the reference beam that was used to store that frame.
As used herein and in the appended claims, the term “frame” refers to a set of data that was stored as an interference pattern in a holographic storage medium using (1) an incident beam of light having a defined incidence angle, or (2) an incident beam of light having a defined wavelength. For example, a frame of an angle-multiplexed holographic storage medium can be stored as an interference pattern in the holographic storage medium using an object beam (e.g., a spherical wave) object beam and reference beam (e.g., a coherent plane wave, such as that from a laser). Superposition of the object beam and the reference beam at the holographic storage medium forms an interference pattern as described above. The data stored in that particular frame can then be read from the holographic storage medium by (1) illumination with the reference beam, which is diffracted by the stored interference pattern to reconstruct the original spherical wavefront of the object beam, (2) illumination with the object beam, which is diffracted by the stored interference pattern to reconstruct the original plane wave reference beam, or (3) a counter-propagating, or phase-conjugate, reference beam, which is diffracted by the stored interference pattern to reconstruct a phase-conjugate copy of the original object beam.